1. Field of the Invention
The present invention is in the field of manufacturing pn junctions and transistors so as to provide a high breakdown voltage at the boundary surface, and employs a specially designed mask to produce a controlled profile of dopant from the main body of dopant to the boundary layer.
2. Description of the Prior Art
There are known methods wherein at the same time that a pn junction is formed, one or more field-limiting ring structures surrounding the pn junction are diffused into the semiconductor body. These ring structures whose penetration depth corresponds to that of the pn junction are disconnected from external potentials during operation and act as voltage dividers, the divider ratio being defined by the mutual spacings and the spacing of the innermost ring structure from the pn junction. In this regard, reference is invited to the book entitled "Thyristor Physics" by A. Blicher, Springer Verlag, 1976, pages 231-234. The disadvantage of these ring structures is that they require a relatively high semiconductor area for achieving high breakdown voltages and are extremely sensitive to surface charges.